RF switches are widely used in modern communication systems. Field effect transistors (FETs) are preferred in many RF switching applications because of their comparatively high OFF impedance, comparatively low ON impedance, low leakage and relatively low drive voltages. One of the most challenging specifications that FET RF switches must meet is for transmit/receive (T/R) switches in the Global System for Mobile Communications (GSM). The power handling, linearity and isolation requirements of the GSM system are very stringent. To meet these challenges, multi-gate FET structures and circuits have been developed. While such implementations are useful they suffer from a number of limitations well known in the art.
FIG. 1 illustrates prior art multi-gate transmit-receive RF switch 10 employing n-channel depletion mode FETs 12, 14 coupled respectively between antenna port 16 and receiver port 18 and transmitter port 20. Resistors Re shunt the source-drain regions of transistors 12, 14. Resistors Rg are in series with the gate leads 13, 15 of transistors 12, 14. Control voltages Vc and Vc(bar) are provided respectively at ports or connections 22, 24. Depletion mode devices 12, 14 are ON at Vgs=0 and OFF when Vgs exceeds the device threshold voltage. DC blocking capacitors (Cblk) 26, 28, 30 are provided so that the source and drain regions of transistors 12, 14 can float with respect to control voltages Vc, Vc(bar). This permits circuit 10 to operate from a single positive supply of, for example, Vc=+3 volts and Vc(bar)=0 volts (and vice versa). Because of the leakage through the transistors, node 33 will tend to drift to the highest DC operating potential. Then, for example, when Vc=+3 volts and Vc(bar)=0 volts, transistor 12 will usually be ON and transistor 14 will be OFF. When the polarity is reversed (i.e., Vc=0 volts and Vc(bar)=+3 volts), then transistor 12 is usually OFF and transistor 14 is ON. However, it is also common to have additional control pin or connection 32 coupled to node 33 in order to supply the desired operating bias to circuit 10. A significant limitation of this prior art circuit and biasing arrangement is that it is not suitable for use with enhancement mode n-channel devices, which are preferred because of their utility and process compatibility with other circuit elements needed to form a single-chip integrated circuit (IC) that includes, for example, amplifiers and other functions along with one or more RF switches. Accordingly there continues to be a need for improved RF switches, especially for multi-gate enhancement mode RF switches suitable for integration with various other circuit elements needed in monolithic integrated circuits (ICs) for mobile communications.
Accordingly, it is desirable to provide an improved RF switching device and method. It is further desirable that the RF switching device be an enhancement mode FET RF switching device, and that it be adaptable for multi-port applications, especially in transmit-receive or equivalent switching applications. Still further, it is desirable that the FET RF switch be capable of operating from a single positive supply and/or switching voltage and biased to an appropriate operating point without the need for other reference voltages and/or additional control pins. In addition, it is desirable that process technology employed for forming the RF switch be compatible with other communication circuit elements desired to be incorporated in monolithic integrated circuits (ICs) for communication applications. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.